The invention relates generally to the manufacture of electronic devices. More specifically, this invention relates to compositions and methods for trimming photoresist patterns useful in the formation of fine lithographic patterns.
In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layer, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. Photoresist materials further find use, for example, in semiconductor manufacturing in the formation of ion implantation masks. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing. Such resists typically employ a resin having acid-labile leaving groups and a photoacid generator. Patternwise exposure to activating radiation through a photomask causes the acid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in exposed regions of the resin. This creates a difference in solubility characteristics between exposed and unexposed regions of the resist in an aqueous alkaline developer solution. In a positive tone development (PTD) process, exposed regions of the resist are soluble in the aqueous alkaline developer and are removed from the substrate surface, whereas unexposed regions, which are insoluble in the developer, remain after development to form a positive image.
Lithographic scaling has conventionally been achieved by increasing the numerical aperture of the optical exposure equipment and use of shorter and shorter exposure wavelengths, for example, 200 nm or less, for example, 193 nm or EUV wavelengths (e.g., 13.5 nm), with chemically amplified photoresists. To further improve lithographic performance, immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The immersion fluid allows a greater amount of light to be focused into the resist layer than would occur with an air or inert gas medium. When using water as the immersion fluid, the maximum numerical aperture can be increased, for example, from 1.2 to 1.35. With such an increase in numerical aperture, it is possible to achieve a 40 nm half-pitch resolution in a single exposure process, thus allowing for improved design shrink. This standard immersion lithography process, however, is generally not suitable for manufacture of devices requiring greater resolution.
At present, the industry has reached a point at which further increases in numerical aperture or reductions in exposure wavelength are not feasible. As a result, alternative methods of scaling integrated circuit lithography are being investigated. Considerable effort has been made to extend the practical resolution beyond that achieved with standard photolithographic techniques from both a materials and processing standpoints. For example, multiple (i.e., double or higher order) patterning processes have been proposed for printing CDs and pitches beyond lower resolution limits of conventional lithographic tools. One such double patterning process is litho-litho-etch (LLE) double patterning, which involves formation of a first lithographic photoresist pattern followed by formation of a second lithographic photoresist pattern, wherein lines of the second pattern are disposed between adjacent lines of the first pattern. LLE double patterning and other advanced lithographic processes often require the formation of isolated features such as lines or posts by direct lithographic printing. The formation of isolated features with an acceptable process window, however, can pose a challenge as a result of poor aerial image contrast at defocus.
To form finer photoresist patterns than attainable by direct imaging alone, photoresist pattern trimming processes have been proposed (see, e.g., U.S. Patent Application Pub. Nos. US2013/0171574A1, US2013/0171825A1, US2014/0186772A1 and US2015/0202414A1). Photoresist trimming processes typically involve contacting a photoresist pattern that includes a polymer having acid labile groups with a composition containing an acid or acid generator. The acid or generated acid causes deprotection in a surface region of the resist pattern, which region is then removed, for example, by contact with a developer solution. The features of the resulting resist pattern are therefore reduced in size as compared with the original resist pattern.
Photoresist trimming processes can suffer from iso-dense bias, by which a difference in dimensions of isolated resist features as compared with dimensions of more densely packed resist features following the trimming process. As a result of the differing dimensions of the resist patterns and subsequently etched features, properties such as electrical characteristics of the resulting devices can be detrimentally affected. Iso-dense bias can, for example, result in a bimodal distribution of conductivity, which can also adversely impact device performance. Without wishing to be bound by any particular theory, the inventors believe that the problem is a result of an increased presence of acid in the isolated resist pattern regions available for deprotection of the resist patterns as compared with the more densely packed resist pattern regions. As such, increased deprotection of the isolated resist patterns can occur due to a larger amount of acid penetrating further into the isolated pattern surface. Iso-dense bias provides an indication of whether an existing photomask can be used to print isolated and dense patterns on the mask without the need for Optical Proximity Correction (OPC). If OPC is required, a new photomask is typically required. It would be desirable to reduce or avoid the occurrence of post-trim iso-dense bias.
There is a need in the art for thermal acid generators, trimming compositions and trimming methods useful in electronic device fabrication that address one or more problem associated with the state of the art.